In our earlier filed application (WO2012142654) we described a magnetometer useful for detecting micro-magnetic fields. The magnetometer comprised a microcavity having optical modes and mechanical modes. A tunable laser was tuned to produce optical radiation at a frequency locked to an optical mode of the microcavity and was evanescently coupled into the microcavity. A magnetostrictive material was attached to the microcavity so that a change in the dimensions of the magnetostrictive material under the influence of a magnetic field was translated to stress in the microcavity causing a change in the mechanical modes of the microcavity. The change in the mechanical modes were detectable on the optical radiation.
Another example of a microresonator coated with a magnetostrictive material is described in U.S. Pat. No. 8,125,644 assigned to Raytheon Corporation. The Raytheon device detects changes in optical mode as a result of changes in the optical cavity caused by a change of dimension of the magnetostrictive material in a magnetic field. What is measured is a shift in the frequency or wavelength) of the resonant optical mode.
These devices are attractive because they are useful in a range of applications requiring measurement of very weak magnetic fields, such as neural mapping. As explained in WO2012142654, the current preferred devices for detecting magnetic fields in the femtoTesla to picoTesla range are Superconducting Quantum Interference Devices (SQUIDS) but these devices have significant handling disadvantages.
Another optical approach to magnetometry, using nitrogen vacancy (NV) centres in diamond as demonstrated in D. Le Sage, L. M. Pham, N. Bar-Gill, C. Belthangady, M. D. Lukin, A. Yacoby, and R. L. Walsworth “Efficient photon detection from color centers in a diamond optical waveguide” Physical Review B, 85, 121202(R) (2012), has also recently gained traction. This approach can achieve excellent sensitivity down to 100 pT, but the diamond substrate presents significant challenges for integration in a chip-based architecture. Additionally, the bandwidth is limited to about 2 MHz, which could limit sensitivity to magnetic resonance signals (MRI).
Optical magnetometry (using resonant cavities coated with magnetostrictive materials) have shown potential for a range of applications. Devices with better sensitivity are needed for optical magnetometry to be useful for microfluidic magnetic resonance imaging, neural imaging, and study of interesting systems like spin physics in condensed matter.